This "hub" scientific component provides the biophysical modeling background necessary for the design, analysis, and interpretation of the quantitative experiments in the other components of this Program Project. The radiobiological information obtained will be used to interpret existing epidemiological data, to produce improved extrapolations to domestic radon exposure situations. Assessment of the current main ares of uncertainty in extrapolating from miner-based risk estimates to domestic risk estimations yields 1) extrapolation from he effects of multiple a-particle traversals (high doses) to single a-particle traversals (low doses); 2) interaction of a- particle damage with that from other lung carcinogens, and 3) effects of a- particle exposure time. The main uncertainties in extrapolating from A- bomb survivor rick estimates to domestic radon risk estimates are the above, as well as 4) radon-progeny quality factors. In the longer term, if further epidemiological research is to be undertaken then 5) development of a biological marker characteristic of radon-induced damage, would have the potential to increase considerably the power of such studies. Experimental design in Projects 2 to 4 is intended to focus on these areas: Here, the intent is to use the conclusions obtained in these analyses, particularly in regard to single-particle effects, agent interaction, dose protraction, and quality factor, to reassess the analysis and extrapolation of miner-based epidemiological data, to provide lifetime radon risk assessments under domestic conditions. Specific areas considered will be 1) design and modeling of experiments with the single-particle microbeam; 2) design and modeling of agent-interaction experiments; 3) design and modeling of experiments relating to alpha-particle dose protraction; 4) design and modeling of alpha-particle RBE experiments: 5) design and modeling of radon "biomarker" experiments. Data will be used from the recent 11 miner-cohort NCI study, in contrast to previous analyses, extrapolations to the domestic environment will be made using radiobiological models derived in this project, for dose, dose rate, and other agent interactions. Finally, realistic quality factors obtained here will be applied to risk estimates obtained based on A-bomb data, to investigate consistence between the epidemiological and "dosimetric" approaches to domestic radon risk estimation.